In commercial operations, it has been found that the more processable butyl rubbers possess sufficient green strength to resist excessive flow and deformation in various handling procedures. It is generally believed that green strength is related to molecular weight with green strength improving as the molecular weight increases. However, it is also desirable in certain applications such as the manufacture of tires, that the rubbers have a rapid stress relaxation rate so that the stresses imposed during the manufacturing process relax quickly and the rubber does not slowly change its shape or pull apart due to the undissipated stresses. However, as the stress relaxation rate is also a function of molecular weight with the relaxation rate becoming slower as the molecular weight increases, when the molecular weight is increased to improve green strength, the stress relaxation rate is reduced. Thus, as the butyl rubber becomes better able to resist flow and deformation in various handling procedures, it becomes more prone to change shape or pull apart due to the unrelaxed stresses. Hence, in the manufacture of butyl rubbers, there is a necessity to strike a balance between the higher molecular weight needed to achieve increased green strength and the lower molecular weight required to bring about a reduction in the stress relaxation rate.
Various means have been used for modifying the molecular weight and/or the molecular weight distribution of butyl rubbers. One means described in U.S. Pat. No. 5,071,913 (Exxon) is by blending butyl rubbers and polyisobutylenes of widely differing and defined molecular weights to produce "tailormade" molecular weight distributions. This has resulted in polymers and polymer compositions with the unique combination of green strength levels attributable to higher molecular weight butyl polymers combined with lower viscosity and faster stress relaxation attributable to lower molecular weight butyl polymers.
Another means of modifying the molecular weight and/or the molecular weight distribution of butyl rubbers, also detailed in U.S. Pat. No. 5,071,913, is achieved by introducing selectively and in a controlled manner during the polymerization cationically active branching agents such as polyisoprene, block copolymers of polybutadiene and polystyrene and their partially hydrogenated or hydrohalogenated derivatives. A small amount of a very highly functional and reactive soluble moiety can be used to introduce a small amount of very high molecular weight, highly branched material into the distribution. Conversely, a larger amount of a less reactive, lower functionality moiety can be used to introduce more of a lower molecular weight, branched material into the distribution.
A method by which the cold flow characteristics of butyl rubber can be improved is described in U.S. Pat. No. 2,781,334 and involves the use of minor amounts of divinyl aromatic hydrocarbon compounds in the butyl polymerization process to afford a butyl rubber that is partially crosslinked.
Heretobefore, there has not been described a process for the preparation of butyl rubber wherein the use of a catalyst system comprising an initiator component having a specific molecular structure and a Lewis acid in the polymerization process permits the formation of a butyl rubber having a bimodal molecular weight distribution.